Such a method is known from EP-A-0 329 228 and from EP-A-1 727 677.
In the method known from these documents the printing plates to be positioned are provided with at least two specific reference marks. Because the relative position of these reference marks relative to the printing plate are known, the printing plate can be placed in register by means of reference to these reference marks.
In the method known from EP-A-0 329 228 two cameras are used which are both coupled to a monitor. The cameras are positioned such that cross-hairs projected on the monitors coincide with the projection of the reference marks of the printing plate placed in register. Positioning of the monitors can take place manually, but can also take place under automatic control. When the cameras have thus been moved to the correct position, the printing plate is manipulated such that the projection of the reference marks on the monitor coincides with the cross-hairs on the monitor. It is noted that two reference marks are necessary here and that the method requires two cameras.
The method described in EP-A-1 727 677 operates automatically however; the camera is adapted to detect the position of the reference marks on the printing plates with a control member, and the control member calculates the path of the printing plates to the register position. A transport member then displaces the printing plate to the register position in accordance with the calculated path. Use is also made here of two reference marks.
In both cases the method requires a printing plate provided with at least two reference marks, as well as the information as to where these reference marks are located on the printing plate, or the relative position of the reference marks relative to the printing plate. These reference marks are printed onto the printed material. Now that the quality of printed material produced by means of flexo-printing techniques is improving, these reference marks, which can usually also be seen on the printed material, are increasingly perceived as disturbing. These drawbacks are partially obviated by making use of reference marks with very small dimensions, also known as microdots, but an increasing need remains for printing plates no longer provided with reference marks leaving an imprint.